In a semiconductor device, semiconductor elements are formed on a semiconductor substrate, and thereafter a lead layer or a wiring layer is formed to electrically connect these semiconductor elements to each other. Further, the surfaces of these semiconductor elements and the lead layer are covered with a protective insulating film so that the elements and the layer are not influenced by external environmental effects such as moisture or stress, and then the semiconductor device is packaged in a mold resin or ceramic package.
For such a protective insulating film, frequently used is a silicon nitride film which has an extremely small moisture permeability and a high mechanical strength compared to those of a silicon oxide film. The silicon nitride film is formed by a chemical vapor deposition process in a plasma (hereafter referred to as a P-CVD process) with silane (SiH.sub.4) and ammonia (NH.sub.3) as main component gases, and has a refractive index of about 1.90 to 2.10.
However, during the process for forming the silicon nitride film, hydrogen contained in the component gases is not completely removed but remains in the film. Therefore, the film contains much hydrogen. For example, the silicon nitride film has six times more hydrogen concentration than a silicon oxide film which is formed by a chemical vapor deposition process in a plasma with silane (SiH.sub.4) and nitrous oxide (N.sub.2 O) as main component gases, and has a refractive index of about 1.53 to 1.59.
The hydrogen in the above silicon nitride film is easily liberated by a low-temperature heat treatment. Therefore, in a relatively low temperature heat treatment at 300 to 400.degree. C. for 10 to 60 minutes performed for recovery from plasma damage after formation of a protective insulating film, the hydrogen easily reaches the vicinity of a semiconductor element by diffusion. As a result, the hydrogen deteriorates the characteristics of sensitive semiconductor elements such as electrically-erasable nonvolatile memory elements (hereafter referred to as EEPROM elements).
As described above, a silicon nitride film is superior in moisture resistance and mechanical strength. However, since the film contains much hydrogen, it cannot be used as a protective insulating film for a sensitive element such as an EEPROM.
FIG. 3 is a sectional view showing the structure of a conventional EEPROM device. In FIG. 3, the device includes a semiconductor substrate 1 (hereafter referred to as substrate) made of single crystal silicon, an EEPROM element 2 formed on the substrate 1, an interlayer insulating film 3 made of a BPSG film formed on the whole surface of the EEPROM element 2, a lead layer or wiring 4 made of aluminum formed on the interlayer insulating film 3, and a silane-based silicon oxide film 5 serving as a protective insulating film formed to wholly cover the surfaces of the EEPROM element 2 and the lead layer 4. The silane-based silicon oxide film 5 is formed up to a thickness of about 1.2 .mu.m by a P-CVD process with silane (SiH.sub.4) and nitrous oxide (N.sub.2 O) as main component gases, and has a refractive index of about 1.53 to 1.59.
As described above, a conventional EEPROM device employs a silane-based silicon film 5 formed by the P-CVD process as a protective insulating film. However, the silane-based silicon film 5 has a low step coverage characteristic because it is formed mainly in a vapor phase. Therefore, as shown in FIG. 4, the silane-based silicon film 5 causes an overhang shape 6 to develop at an end of the lead layer 4, and its thickness decreases between lead layers 4, particularly at a stepped portion 7.
Further, the silane-based silicon film 5 is inferior to a silicon nitride film stated above in mechanical strength and in blocking characteristic for permeation of moisture or the like. Therefore, problems arise in that the quality of the silane-based silicon oxide film 5 deteriorates at the stepped portion 7 having reduced thickness. Thus, moisture permeates through this portion to reach the lower-laying interlayer insulating film 3, which causes imperfect insulation of the interlayer insulating film 3 and current leakage between lead layers 4.